1. Field of the Invention
The present invention relates to a low resistance ohmic electrode for semiconductors composed of III--V group compounds having a p-type conductivity (hereinafter, such a semiconductor will be referred to as a "p-type III-V compound semiconductor"), and a bipolar transistor incorporating the ohmic electrode. The present invention relates to a method for producing the ohmic electrode end a method for producing the bipolar transistor.
2. Description of the Related Art
As a low resistance ohmic electrode for p-type GaAs layers, a Pt/Ti/Pt/Au electrode is drawing much attention (H. Okada, et al., Japanese Journal of Applied Physics Vol. 30, 1991, pp. L558-L560). This ohmic electrode includes a Pt film, a Ti film, a Pt film, and an Au film layered on a p-type GaAs substrate in this order. Since the Schottky junction of Pt is low relative to that of p-type GaAs, the lowermost Pt layer acts to reduce the contact resistance of the electrode. The Ti layer and the middle Pt layer function to prevent mutual diffusion between the Ga and As and the uppermost Au layer.
By using an electrode of this structure as a base electrode for a bipolar transistor, and subjecting it to a heat treatment at 350.degree. C., an extremely low base resistance can be obtained, thereby making the high frequency characteristics of the bipolar transistor excellent. This is reported in Extended Abstracts of the 1993 International Conference on Solid State Devices and Materials, pp. 1062-1064.
On the other hand, an electrode of an Ni/Ti/Ag structure is known as a low-contact resistance electrode for n-type Si layers, as is disclosed in Japanese Laid-Open Patent Publication No. 65-234322.
However, the above-mentioned ohmic electrode has a problem in that, since the low resistance ohmic contact is achieved by conducting an alloying process at a relatively low temperature, the resistance of the ohmic contact increases when the electrode is kept at a temperature exceeding about 350.degree. C. According to an experiment by the inventors of the present invention, as shown in FIG. 10, the contact resistance of the Pt/Ti/Pt/Au electrode becomes minimum when the heat treatment is conducted at about 350.degree. C., and increases at any temperature above 350.degree. C. This is considered to be because compounds, such as PtAs.sub.2, that increases the contact resistance is generated at about 400.degree. C. during the heat treatment. When a p-type semiconductor layer included in the ohmic electrode is as thin as a base layer of a hetero-junction bipolar transistor (HBT), PtAs.sub.2 and the like may be formed deep from the surface of the p-type semiconductor. As a result, the thickness of the p-type semiconductor layer remaining under the PtAs.sub.2 is inevitably reduced, thereby further increasing the contact resistance.
Moreover, the following problem arises during the fabrication of bipolar transistors.
According to an experiment by the inventors, as shown in FIG. 11, n-type AuGe/Ni type ohmic electrodes that are commonly used as collector electrodes of bipolar transistors have such characteristics that the contact resistance thereof becomes minimum at about 400.degree. C. Accordingly, in the fabrication process of a bipolar transistor, it is required to first form a collector electrode, subject the collector electrode to a heat treatment at 400.degree. C., and then form a base electrode and subject the base electrode to a heat treatment at 350.degree. C. so that the contact resistance of a Pt/Ti/Pt/Au base electrode and the contact resistance of an AuGe/Ni type collector electrode are minimized. In other words, it is impossible to form the collector electrode after the formation of the base electrode, creating problems so that the fabrication process of HBTs allows less freedom and that separate heat treatments must be conducted for the base electrode and the collector electrode, respectively.